HOW ARE EARTHQUAKES MEASURED?
Two numbers are commonly given to describe earthquakes — intensity and magnitude. Intensity, also referred to as the “Mercalli scale,” is a number based on people’s observations (or “felt reports”). It is related to the damage caused by local ground shaking and ground failure. This scale ranges from I, which is noticed by almost nobody, up to XII, which includes virtually total destruction of structures. Significant damage to buildings occurs at Mercalli intensities of VII or greater. Intensity decreases with distance from the epicenter, but local soil conditions — for example, the artificial fill near the shore of San Francisco Bay in the Loma Prieta earthquake — can cause higher values at distant sites. Magnitude, commonly referred to as the “Richter scale,” quantitatively describes the strength of an earthquake. It is usually based on the maximum amplitutde of certain types of seismic waves recorded on a seismograph. The Richter scale was originally developed in the 1930s by seis
The size of an earthquake is indicated by a number called its magnitude. Magnitude is calculated from a measurement of either the amplitude or the duration of specific types of recorded seismic waves. Magnitude is determined from measurements made from seismograms and not on reports of shaking or interpretations of building damage. In general, the different magnitude scales (for example, local or Richter magnitude and surface wave magnitude) give similar numerical estimates of the size of an earthquake, and all display a logarithmic relation to recorded ground motion. That means each unit increase in magnitude represents an increase in the size of the recorded signal by a factor of 10. Therefore, a magnitude 7 earthquake would have a maximum signal amplitude 10 times greater than that of a magnitude 6 earthquake and 100 times greater than that of a magnitude 5 earthquake. Seismologists sometimes refer to the size of an earthquake as moderate (magnitude 5), large (magnitude 6), major (m
A 10–fold increase in amplitude represents about a 32–fold increase in energy released for the same duration of shaking. The best known magnitude scale is one designed by C.F. Richter in 1935 for west coast earthquakes. In New Jersey, earthquakes are measured with seismographs operated by the Lamont–Doherty Earth Observatory of Columbia University and the Delaware Geological Survey. An earthquake’s intensity is determined by observing its effects at a particular place on the Earth’s surface. Intensity depends on the earthquake’s magnitude, the distance from the epicenter, and local geology. These scales are based on reports of people awakening, felt movements, sounds, and visible effects on structures and landscapes. The most commonly used scale in the United States is the Modified Mercalli Intensity Scale, and its values are usually reported in Roman numerals to distinguish them from magnitudes. TOP Past damage in New Jersey New Jersey doesn’t get many earthquakes, but it does get
A seismometer is an instrument that senses the earth’s motion; a seismograph combines a seismometer with recording equipment to obtain a permanent record of the motion. From this record scientists can calculate how much energy was released in an earthquake, which is one way to decide its magnitude. Calculations are made from several different seismograms, both close to and far from an earthquake source to determine its magnitude. Calculations from various seismic stations and seismographs should give the same magnitude, with only one magnitude for any given earthquake. To determine the strength and location of earthquakes, scientists use a recording instrument known as a seismograph. A seismograph is equipped with sensors called seismometers that can detect ground motions caused by seismic waves from both near and distant earthquakes. Some seismometers are capable of detecting ground motion as small as 1 billionth of a meter, or about 40 billionth of an inch. A seismograph produces wav
